vm_xml: let delete device can also by device tag

In some situations, we don't care the device xml, so after this patch,
we can avoid generate device xml by set 'by_tag=True'.

Signed-off-by: Yanbing Du <ydu@redhat.com>
vm_xml: Add memtune XML support and add 'unit' attribute for memory element

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1848 from will-Do/memtune_xml

vm_xml: Add memtune XML support
Merge pull request #1852 from will-Do/del_device

vm_xml: let delete device can also by device tag
virttest.utils_misc: Use RLock to protect _open_log_files

Signed-off-by: Feng Yang <fyang@redhat.com>